Land Ocean Coupling Coupling riverine fluxes of nutrients to a Global Biogeochemical Ocean General Circulation Model Christophe Bernard, Christoph Heinze Geophysical Institute, Bjerknes Center for Climate Research University of Bergen

NPZD model with colimitation of Nutrients such as N, P,Si,Fe orthogonal curvilinear C-grid with a formal resolution of 3◦ 20 km in the Arctic and about 350 km in the Tropics 40 vertical levels with level thickness increasing with depth North pole is located over Greenland and the other over Antarctica. The HAMOCC5-MPIOM

Application of the coastal segmentation : estimating natural silica fluxes to the coastal zone Dürr and Meybeck, 2007 Relative silica flux Global mean silica yield 3.3 t SiO 2 km -2 yr -1 Hyperactive Very active Active Sub-active Hypo-active Inactive ‘Dead’ > 50% of natural silica yield 6,2 teramoles of silicon per year

The riverine inputs Figure 1. Integrated annual flux of silica as added in the model grid, according to the 129 coastal segments from the COSCAT approach. Riverine silica inputs are given in megamoles Si per year.

The coupling Flux computed at each time step: 10 times a day It includes : Si, DIN, DIP, DON,DOP,PP,PN and POC Homogeneous along the coastal segment Constant over time

With riverine silicate Without riverine silicate Computing the difference between the 2 runs Riverine contribution to the Opal export production

Fate of riverine Si depends on the level of primary production. Limiting nutrientAnnual photosynthesis the computation of the photosynthesis is driven by the less available nutrient corrected by a multiplying factor. Bernard et al. 2008, submitted

Example of the Amazon contribution Figure 4. The seasonal cycle of nutrient limitation (element equivalent phosphorous) in the Amazon plume, (lower panels) with (left) and without (right) riverine silica inputs. Opal and calcium carbonate export (F opal and F calcarb) at 10m depth in response to photosynthesis (upper panels). Nutrient limitation is expressed as the concentration adjusted to the necessary stoichiometric concentration of nitrogen and iron relative to phosphate. The limitation of photosynthesis is driven by the lowest concentration equivalent phosphate (iron, nitrate or phosphate). Opal and calcium carbonate competition is driven by the silica concentration. Bernard et al. 2008, submitted

Continental margins in the model’s grid Defined as the 8% shallowest grid points

Marge vs global ocean: the riverine nutrients contribution Carbon Silica 0.5 Gt C

Relative contribution of riverine nutrients to the export production of Opal and C All NutNo NutNo SiNo Org No Part No DINNo DIP C Si

Marginal seas…. All NutNo NutNo SiNo Org No Part No DINNo DIPNo CAll DI form 0,09 Gt C

To summarise… and conclude. Human activity (urban development, land use, damming) changes the river load of nutrients to the ocean(decreased Si, increased N and P). Changes in the riverine inputs of nutrients do have an impact on a global scale. Marginal seas are more sensitive to river load changes. Eutrophication leads to a larger burial of Opal on the continental shelf.